The present invention relates to an improved method of washing the media bed of a gas pollution abatement reactor, such as a regenerative thermal oxidizer or a selective catalytic reduction system, and a bed washer apparatus.
Regenerative thermal oxidizers or RTOs are now widely used for oxidizing gaseous pollutants including volatile organic compounds, such as hydrocarbons, in waste or exhaust gas streams. A typical regenerative thermal oxidizer includes at least two heat exchange chambers each having a heat exchange media bed therein and a combustion chamber located above the heat exchange media communicating with the heat exchange chambers. The waste gas stream is directed alternatively or periodically into one of the heat exchange chambers which has been previously heated and wherein the gaseous pollutants are oxidized. The gas then flows into the combustion chamber, wherein any remaining gaseous pollutants are oxidized. The combustion chamber is also used to preheat the gas flowing through the regenerative thermal oxidizer during start-up and to oxidize any remaining pollutants in the waste gas stream. The cleansed heated gas then flows into the second heat exchange chamber, heating the media bed in the second heat exchange chamber and the cleansed gas from the second media bed is vented to atmosphere. The gas flow through the regenerative thermal oxidizer is then reversed, such that the waste gas stream flows into the heat exchange media in the second heat exchange chamber, oxidizing the pollutants, etc. A regenerative thermal oxidizer thereby conserves heat resulting in a more efficient gas pollution abatement system.
A regenerative thermal oxidizer of the type described herein may include two or three heat exchange chambers, wherein the third chamber serves as a purge chamber. A series of control valves then directs the gas through the heat exchange chambers as described above. By alternating the flow through the regenerative thermal oxidizer through the heat exchange chambers, the pollutants in the process or exhaust gas is removed and oxidized without exhausting pollutants to the atmosphere and the heat exchange media is periodically cleaned.
The heat exchange media in the media bed may comprise relatively small ceramic elements, generally saddle shaped ceramic elements, or the media bed may be formed of stacked ceramic blocks each having small continuous passages therethrough. The ceramic media may also include or be coated with a catalyst resulting in a catalytic reaction within the chamber to remove gaseous pollutants. Where the media includes a catalyst, the gaseous pollutant abatement system is generally referred to as a selective catalytic reduction apparatus or SCR system used primarily to treat NOx, including NO and NO2. The ceramic media may be coated with a suitable catalyst or the catalyst may be mixed with the ceramic matrix prior to firing. Typical catalysts include noble metal catalysts, such as platinum, and base metal catalysts, such as vanadium or manganese oxide or Zeolite. A typical SCR system includes only one reaction chamber filled with a catalytic media bed as described. The gas to be treated flows through the bed of catalytic media in the reaction chamber where the NOx is reduced to nitrogen gas and non-polluting oxides.
As used herein, the term xe2x80x9cgas pollution abatement reactorxe2x80x9d is intended to include both RTOs and SCRs and other gas pollution abatement apparatus having a media bed, wherein the media bed is heated to oxidize or react with the gaseous pollutants, thereby removing the pollutants prior to venting the gas to atmosphere. However, in many real world applications of these systems, the industrial process gas emissions further contain solid particulate material in addition to the gas phase pollutants the abatement reactor is intended to destroy. These particulates can accumulate in the media bed in sufficient quantities such that the accumulated particulate material will cause an increase in the airflow resistance through the media bed, increasing the pressure drop across the media bed, thereby restricting the airflow capacity of the system and preventing the process equipment from operating properly. Because these particulates are endemic to many real world applications and they can cause the gas pollution abatement reactor to become inoperative, techniques have been developed to clean these particulates from the media bed.
The presently preferred method of cleaning particulates from the media bed is water washing. Water washing is used primarily to clean non-burnable particulate accumulations from RTOs and non-reactive particulates from SCRs. Burnable particulates are typically cleaned from the media bed of an RTO using a xe2x80x9cbake outxe2x80x9d technique. Conventional water washing is accomplished by the following procedure. First, the gas pollution abatement reactor is taken xe2x80x9coffline,xe2x80x9d shut down and cooled to ambient temperature. The access door located above the media bed is then opened and the atmosphere is checked as required for personnel entry. The media bed is then washed, typically using a fire hose connected to a supply of wash water. The wash water is then sprayed over the media bed by personnel standing on the media bed who manually distribute the wash water over the media bed by moving the hose from place to place. After the washing is completed, the media bed is dried and reheated prior to placing the gas pollution abatement reactor back in service.
There are several important disadvantages of this conventional washing technique. First is worker safety. If there are any xe2x80x9chot spotsxe2x80x9d in the media bed when the washing is started and the water is directed onto these hot spots, steam will be generated and released. This can cause hazardous temperatures. In addition, the steam can fog the workers"" eyewear, making it difficult for them to exit. Depending on how the reaction media is cooled, these hot spots may be well below the upper surface of the media and not apparent upon inspection. Another disadvantage of this water washing technique is the potential for the workers to wet or damage the ceramic fiber insulation inside the combustion chamber of an RTO. If this ceramic fiber insulation is wetted, it can sag or shrink, creating gaps which will lead to hot spots on the outside skin of the reaction chamber. These hot spots can lead to unnecessary heat loss, cosmetic damage to the exterior finish of the housing, and possible corrosion of the outer shell. Another disadvantage of this method is the potential for poor distribution of the wash water over the surface of the media bed. Because the workers must manually move the hose from place to place, there is the potential for some places being missed and other places washed more than necessary. Thus, this method is wasteful of wash water and can lead to incomplete washing.
There is, therefore, a long felt need to improve the method of washing the media bed of a gas pollution reactor such as an RTO or SCR which assures personnel safety, reduces damage to the ceramic fiber lining of the housing, and assures even distribution of wash water over the media bed. The method of washing the media bed of a gas pollution abatement reactor and bed washer apparatus of this invention solves these problems by eliminating the need of a worker being located within the reactor housing during washing, and by utilizing a bed washer which assures even distribution of the wash water over the media bed without spraying the ceramic fiber installation inside the housing.
As set forth above, the present invention relates to a method of washing the reaction media bed of a gas pollution abatement reactor and an apparatus or bed washer for washing the media bed which eliminates the requirement for personnel to be located within the housing during washing and which assures even distribution of the wash water over the surface of the media bed without spraying the insulation during washing. A typical gas pollution abatement reactor of the type described above includes a housing having generally parallel side walls, opposed end walls and an access door located above the media bed. The media bed may be comprised of relatively small particles of ceramic media or stacked ceramic blocks. The end walls of the housing are typically semicircular or arcuate, but the chamber located above the media may also be rectangular having planar end walls. A typical reaction chamber has parallel side walls and semicircular end walls having a width of about 9 to 12 feet and a length of 15 to 30 feet. The housing further has an access door typically two foot by two foot located above the top surface of the media bed by one to two feet.
The method of washing the reaction media bed of a pollution abatement reactor of this invention includes the following steps. First, the pollution abatement reactor is shut down and the media bed is cooled to generally ambient temperature. Second, the access door is opened and the atmosphere is checked as required for personnel entry as described above. Third, a bed washer is received through the access door and assembled on the top surface of the media bed. In the disclosed embodiment of the bed washer, the bed washer is easily disassembled sufficiently to be received through the relatively small access door of the housing, but the bed washer may also be folded. The bed washer includes a tubular manifold having a plurality of downwardly directed spaced nozzles and a drive wheel for directing the manifold to traverse the media bed. Once the bed washer is assembled, the manifold is connected to a source of washing liquid under pressure and the personnel then exit the housing through the access door prior to washing the media bed. The bed washer is then operated by personnel located outside the housing to traverse the media bed and the media bed is uniformly washed by the washer liquid directed through the downwardly directed nozzles as the bed washer traverses the media bed. In the event that the media bed includes hot spots, the workers are not subjected to steam and the wash water is evenly distributed over the media bed without spraying the side walls of the housing which typically include ceramic insulation as described above. The bed washer is then removed from the housing through the access door by either disassembling the bed washer or folding. After the washing is complete, the media bed is dried and the pollution abatement reactor is activated to heat the media bed to place the reactor back in service.
The preferred embodiment of the bed washer as described above includes a continuous tubular manifold having generally parallel side portions, opposed end portions and a plurality of spaced downwardly directed nozzles directing wash water under pressure on the surface of the media bed. In the most preferred embodiment, the bed washer includes directional control switches at the opposed end portions of the manifold connected to a motor which drives the drive wheel, such that the bed washer reverses direction when the directional control switches engages an end wall of the gas pollution abatement reactor. In the most preferred embodiment, the width of the bed washer measured between the side portions of the manifold is generally equal to the width of the housing such that the entire surface of the media bed is washed with each pass of the bed washer. The side portions of the tubular manifold may also include rub rails, which may be adjustable to accommodate variations in the width of the housings. In the disclosed embodiment, the continuous tubular manifold is supported by a frame extending generally perpendicular to the side portions of the tubular manifold and the drive wheel is rotationally supported by the frame. A motor is connected to the drive wheel and the motor is connected to the directional control switches, which are pneumatic in the disclosed embodiment, to reverse the direction of the bed washer as described. The tubular manifold is also supported by a plurality of stabilizer wheels. The bed washer assembly may be easily disassembled for receipt of the bed washer through the access door of the housing by disconnecting union connections between the end and side portions of the manifold which may be flexible to permit folding of the manifold.
In the most preferred embodiment of the bed washer and method of this invention, the end portions of the tubular manifold are configured to be received in the end walls of the housing. Thus, where the end walls of the housing are semi-circular, the end portions of the tubular manifold are also semi-circular. Alternatively, where the end walls of the housing are planar, the end portions of the tubular manifold are also planar.
The method of washing a media bed of a gas pollution abatement reactor of this invention thus eliminates the problems associated with the current washing method. The workers are located outside the housing when the media bed is washed, eliminating concerns regarding hot spots and the resultant steam, as described above. Further, the media bed is uniformly washed without spraying the side walls of the housing which are normally insulated with ceramic fiber insulation as described above. The bed washer of this invention may be easily disassembled or folded for insertion into the housing through the relatively small access door located above the media bed and the washing procedure may be accomplished in less time than the conventional method described above. The wash liquid will dissolve salts of sodium and potassium chloride for example, entrained in the exhaust gas received by the pollution abatement reactor and wash small particulate material through the media bed which may be collected below the bed. Other advantages and meritorious features of the present invention will be more fully understood from the following description of the preferred embodiments, the appended claims and the drawings, a brief description of which follows.